1. Field of the Invention
The present invention generally relates to illuminating a specimen for inspection or metrology. Certain embodiments relate to reducing the coherence of pulses of light for inspection and metrology applications.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. Metrology and inspection processes are used at various steps during a semiconductor manufacturing process to monitor and control the process. Metrology processes are used to measure one or more characteristics of the wafers. For example, metrology processes are used to measure one or more characteristics of a wafer such as a dimension (e.g., line width, thickness, etc.) of features formed on the wafer during a process such that the performance of the process can be determined from the one or more characteristics. In addition, if the one or more characteristics of the wafer are unacceptable (e.g., out of a predetermined range for the characteristic(s)), the measurements of the one or more characteristics of the wafer may be used to alter one or more parameters of the process such that additional wafers manufactured by the process have acceptable characteristic(s).
Inspection processes are used to detect the presence of defects such as patterning defects and particles on the surface of wafers. For example, inspection processes can include imaging a region of a wafer at high magnification and then comparing that image, e.g., with 1) images of one or more other regions that are supposed to contain the same pattern or 2) a theoretical image, in order to detect differences in the image that may represent defects such as defects in the pattern and particles on, or embedded in, the surface of the wafer. A defect or particle that is smaller than the image resolution of the inspection system optics can often be detected by a change in the reflected or scattered light caused by that defect or particle. Typically, in an inspection process, it is desired to inspect a high percentage, or even 100%, of the wafer surface. Since each high magnification image covers only a small fraction of the wafer surface area, many such images have to be acquired in order to cover the total area that is to be inspected.
In general, metrology and inspection processes can take a relatively long time, particularly when the number of sites on the wafers at which measurements or inspections are performed is relatively large. One obstacle to reducing the time in which metrology and inspection processes can be performed is the substantial difficulty of reducing the time involved in moving the wafer and/or system optics such that multiple sites on the wafer can be measured or inspected. Therefore, one approach to decreasing the time involved in metrology and inspection processes involves continuously moving the wafer and/or system optics relative to one another during the metrology or inspection process. However, such an approach significantly reduces the amount of time in which the measurement can be performed or an image can be captured. Therefore, such an approach requires a light source that can produce a sufficient amount of light in a substantially short period of time. One such light source is a pulsed laser light source. Such light sources have a disadvantage in that the light has a speckle pattern due to the coherence of the light, which can interfere with the metrology measurements, degrade the quality of inspection images, or cause spurious intensity changes in those images. As such, a significant obstacle to using such a light source in a metrology or inspection system that performs measurements or inspection as the wafer and/or system optics are continuously moved relative to one another is that the speckle pattern must be suppressed relatively quickly, and particularly more quickly than the time in which the speckle pattern can be suppressed using mechanical devices, time averaging procedures, or other currently used methods for suppressing speckle patterns.
Accordingly, it would be advantageous to develop illumination methods and/or subsystems for metrology and inspection systems that can provide adequate light with a sufficiently suppressed speckle pattern in a substantially short amount of time.